1. Field of the Invention
This invention relates generally to digital broadcasting. More particularly, it relates to a digital broadcast receiver which is used for a digital broadcasting system in which on the transmission side a broadcast station codes a digital signal to modulate an analog carrier signal for broadcast by radio or by cable, and which on the reception side, a digital broadcast receiver digitally demodulates the received analog signal into a digital signal, detects and corrects errors, and decodes the resulting output back into the original digital signal.
2. Description of the Related Art
Digital signals are typically encoded into a variety of data compression formats to reduce their transmission bit-rate to fit within bandwidth limitations/allocations. Each coded digital signal is assigned a distinct channel that modulates the respective band of an analog carrier signal for broadcast by radio or cable. Terrestrial and satellite radio transmissions, however, are very susceptible to signal distortions caused by local physical effects such as attenuation, cross-talk, and dynamic multipath interference. Cable transmissions may also experience similar problems with signal interference; especially cross-talk induced by magnetic fields from unshielded conductors. These distortions cause digital broadcast receivers to demodulate bits incorrectly from the received analog signal. If uncorrected, single bit errors become significantly magnified when the digital signal is decoded and decompressed. Unfortunately, in a digital broadcast, the data is communicated only in one direction (from broadcast station to receivers) and receivers cannot request retransmissions of the original data when they encounter errors.
Digital broadcasting standards address this problem by specifying forward error correction schemes, which essentially provides redundant coding of the data into parity data to improve the error resiliency of the transmission. The parity data allow receivers to detect and repair errors, and reconstruct the original digital signal. Unfortunately, forward error correction fail sometimes and receivers with more errors than parity data cannot correct any of the errors.
To compartmentalize failures and preserve quality-of-service, the standards also specify digital streaming formats to arrange and transmit the signal data as a sequence of blocks where each block contains its own section of parity data; a burst of errors will limit data erasures to only a few individual blocks, while preserving the remaining parts of the received digital signal. However, a large sustained bit-error rate may preclude many blocks from being decoded and completely interrupt the digital signal reception. In this situation, despite receiving and demodulating some bits correctly, there is not enough information to decode the (compressed) digital signal and the received bits are essentially discarded. Thus, the coverage area of a digital broadcast is effectively limited by the average bit-error rate encountered at local receivers.
Existing approaches to expand this coverage area include increasing the broadcast transmission power to improve the signal gain (e.g. SNR) at local receivers, adding more parity data or coding redundancy to improve error tolerance, or building repeater transmission towers to provide fringe coverage. Unfortunately, these approaches are very costly to implement and do not scale well. Since the broadcasting power is dispersed across a hemisphere, the received signal power is inversely proportional to the squared distance from the transmission source; maintaining the same signal quality while doubling the distance requires quadrupling the original transmission power. Broadcasting with more parity data enables reception at lower SNRs and can extend the broadcast range, but the extra parity data reduces the effective bandwidth of the transmission channel. Finally, building repeater transmission towers involves significant cost, planning, and maintenance, and their proliferation may lead to further signal interference.
Thus, there is a need for a more cost-effective method to improve the quality-of-service and coverage area of a digital broadcast transmission.